The Fluid Mosaic Model of Membranes.pptx

Mosaic fluidic structure of the membrane . The semi permeable and selective permeability of the plasma membrane is explained

1. The Fluid Mosaic Model of Membranes

2. Cell membrane Membranes are vital structures found in all cells The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment, and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and RER Membranes do not only separate different areas but also control the exchange of material across them, as well as acting as an interface for communication Membranes are partially permeable Substances can cross membranes by diffusion, osmosis and active transport

3. The fluid mosaic model of the membrane was first outlined in 1972 and it explains how biological molecules are arranged to form cell membranes The fluid mosaic model also helps to explain: Passive and active movement between cells and their surroundings Cell-to-cell interactions Cell signalling

4. Phospholipids Phospholipids structurally contain two distinct regions: a polar head and two nonpolar tails The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water If phospholipids are spread over the surface of water they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water This is called a phospholipid monolayer

5. phospholipid

6. Chemical Composition of Plasma Membrane: Lipid The lipids identified in the plasma membrane consist of cholesterol, phospholipids and galactolipids The phospholipids include phosphatidylcholine, phosphatidylethanolamine and sphingomyelin . The phospholipids are found to be associated with the outer protein shell in the plasma membrane. Glycerol and fatty acid constitute lipid molecules.

7. In the membrane the lipid molecules consist of two parts —a head and two tails. The head is composed of glycerol and is hydrophilic where as the tails are composed of fatty acids that are hydrophobic.

8. The proteins, in the membrane, are present in two layers and the lipids occur in between them.

9. Protein In the membrane it is present as enzyme protein, carrier protein and structural protein. The enzyme proteins have catalytic activity. The carrier proteins help to transport materials in and out of the cell across membrane. The structural proteins play an important role to form the structure of membrane.

10. Carbohydrates Carbohydrate occur in the form of glycolipids and glycoproteins. The main difference between glycolipid and glycoprotein is that glycolipid is a carbohydrate-attached lipid whereas a glycoprotein is a carbohydrate-attached protein. Glycolipids serve as cell markers or antigens recognized by the immune system as self or non-self while glycoproteins serve as receptors for chemical signals and play a role in cell adhesion.

11. A glycolipid is a lipid molecule attached to a short chain of carbohydrates. Glycolipids occur on the cell membrane of eukaryotes and the sugar residue of the glycolipid molecule extends out from the phospholipid bilayer. The main function of glycolipids is to stabilize the cell membrane by making hydrogen bonds with surrounding water molecules. They involve in cellular recognition, which is important in triggering an immune response. G lycolipids play a key role in cell attachment during the formation of tissue. What is a Glycolipid

12. What is a Glycoprotein A glycoprotein is a protein molecule attached to a short chain of carbohydrates. Generally, glycoproteins are produced as co-translational or post-translational modifications in a process known as glycosylation

13. Phospholipid bilayers can form compartments – the bilayer forming the cell surface membrane establishing the boundary of each cell Internally, membrane-bound compartments formed from phospholipid bilayers provide the basic structure of organelles, allowing for specialisation of process within the cell An example of a membrane-bound organelle is the lysosome (found in animal cells), each containing many hydrolytic enzymes that can break down many different kinds of biomolecule These enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components

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15. Cell Signalling Cell signalling is the process by which messages are sent to cells Cell signalling is very important as it allows multicellular organisms to control / coordinate their bodies and respond to their environments Cell signalling pathways coordinate the activities of cells, even if they are large distances apart within the organism The basic stages of a cell signalling pathway are: A stimulus or signal is received by a receptor The signal is converted to a ‘message’ that can be passed on – this process is known as transduction The ‘message’ is transmitted to a target (effector) An appropriate response is made

16. Transmission of messages in cell signalling pathways requires crossing barriers such as cell surface membranes Cell surface membranes are therefore very important in signalling pathways as the membrane controls which molecules (including cell signalling molecules) can move between the internal and external environments of the cell Signalling molecules are usually very small for easy transport across cell membranes Typically in cell signalling pathways, signalling molecules need to cross or interact with cell membranes

17. Ligand Signalling molecules are often called ligands Ligands are involved in the following stages of a cell signalling pathway: Ligands are secreted from a cell (the sending cell) into the extracellular space The ligands are then transported through the extracellular space to the target cell The ligands bind to surface receptors (specific to that ligand) on the target cell These receptors are formed from glycolipids and glycoproteins The message carried by the ligand is relayed through a chain of chemical messengers inside the cell, triggering a response

18. Diffusion & Facilitated Diffusion Diffusion is a type of transportation that occurs across the cell membrane It can be defined as: The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration. The molecules or ions move down a concentration gradient The random movement is caused by the natural kinetic energy of the molecules or ions

19. Facilitated diffusion Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include: Large polar molecules such as glucose and amino acids Ions such as sodium ions (Na+) and chloride ions ( Cl –) These substances can only cross the phospholipid bilayer with the help of certain proteins This form of diffusion is known as facilitated diffusion There are two types of proteins that enable facilitated diffusion: Channel proteins Carrier proteins They are highly specific (they only allow one type of molecule or ion to pass through)

20. Channel proteins Channel proteins are water-filled pores They allow charged substances ( eg . ions) to diffuse through the cell membrane The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore This allows the channel protein to control the exchange of ions

21. Carrier proteins Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

22. Osmosis The Process of Osmosis All cells are surrounded by a cell membrane which is partially permeable Water can move in and out of cells by osmosis Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane In doing this, water is moving down its concentration gradient The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

23. Active Transport The Process of Active Transport Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion) Although facilitated diffusion also uses carrier protein, active transport is different as it requires energy The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane The energy required is provided by ATP (adenosine triphosphate) produced during respiration

24. Active transport Importance Active transport is important in: Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules Absorption of some products of digestion from the digestive tract Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant Loading inorganic ions from the soil into root hairs

25. Endocytosis & Exocytosis The processes of diffusion, osmosis and active transport are responsible for the transport of individual molecules or ions across cell membranes However, the bulk transport of larger quantities of materials into or out of cells is also possible Examples of these larger quantities of materials that might need to cross the membrane include: Large molecules such as proteins or polysaccharides Parts of cells Whole cells eg . bacteria Bulk transport into cells = endocytosis Bulk transport out of cells = exocytosis These two processes require energy and are therefore forms of active transport